The invention relates to a hip joint prosthesis with an outer metal shell, an inner socket insert in which a ball head articulates, the ball head is joined to a metal prosthesis shaft of the femoral shaft component, and the movement of the prosthesis shaft is limited by abutting against the margin of the socket insert.
In today""s prostheses the movement of the femoral shaft component is limited by measures taken on the hip joint socket. There are various embodiments, e.g.:
A raised metal edge
A raised PE edge (PE=polyethylene) in sandwich systems (e.g., U.S. Pat. No. 5,879,404)
The PE, ceramic or metal insert itself.
In this case the prosthesis shaft is mostly not optimally matched to its counterpart. In shaft prostheses, three positions of the prosthesis shaft with respect to the hip joint socket are often possible.
These said structural designs results in various disadvantages:
Optimization of the shaft component in regard to abutment cannot be achieved. Therefore no adaptation of the abutment geometry to minimize the surface pressure is possible. Furthermore, any adaptation of the shaft component to all possible installation positions is likewise impossible.
When the metal shaft abuts against a metal or ceramic insert, metal detritus is released which can lead to the known negative effects.
In the case of abutment against PE, flow of the PE cannot be avoided. The implant becomes unstable and severe PE deformation can lead to situations wherein the functionality of the implants is not assured.
The PE in the hip joint socket is not optimized for cushioning shaft abutment, since its material and shape are optimized for use as parts in frictional contact with one another. Above all, no defined cushioning of the abutment is possible. In the event of elevated impulse energies this can result in the failure of an implant.
The invention is addressed to the problem of improving a hip joint prosthesis according to the preamble of claim 1 with respect to the abutment of the prosthesis shaft against the socket insert.
This problem is solved by the invention in that a soft, biocompatible cushioning element is applied to the circumferential surface of the prosthesis shaft where the latter impacts the socket insert.
In a preferred embodiment the cushioning element is made from a plastic, e.g., PE (polyethylene), PEI (polyether imide), silicone or TPE (thermoplastic elastomer), to name but a few.
Advantageously, the geometric configuration of the cushioning element is matched to the corresponding edge of the socket insert, i.e., the edge of the socket insert can engage the cushioning element upon abutment, or vice versa.
For this purpose recesses for the corresponding edge of the socket insert are made preferably in the cushioning element""s surface confronting the socket insert. These indentations form a replica of the edge of the socket insert.
It may be advantageous if the axial length of the cushioning element reaches all the way into the ball head. In this way extremely good fixation can be achieved.
In a preferred embodiment, the cushioning element is a ring, advantageously a ring with a slot which can be, for example, axial, at an angle, and/or in a sawtooth manner. The slot makes possible easy assembly of the cushioning element on the prosthesis shaft.
The ends of the slot are advantageously out of line with one another so that cushioning is always achieved wherever abutment occurs.
In a preferred embodiment, cavities are arranged between the cushioning element and the prosthesis shaft to contain body fluids. When the prosthesis shaft strikes the socket insert these cavities are compressed and the body fluid is driven out, so that cushioning is the result.
The idea of the present invention is to apply to the prosthesis shaft a biocompatible layer that is soft in comparison to the metal shaft. The geometrical configuration of this layer is adapted to the corresponding hip joint socket in order to minimize the surface pressures occurring in case of abutment. The structural configuration of the hip joint socket can include a directly clamped design, a sandwich design, a press fit system or a screw-in socket system. The material of this interlayer is preferably a plastic (PE, PEI, silicone, TPE, . . . )
The following are the advantages of this invention over the state of the art:
The system permits optimization as regards an enlarged range of movement (ROM). Abutment optimization has been performed heretofore either on the prosthesis shaft or on the hip joint socket. These defined abutments have been in some cases (raised metal margin) obtained with a limitation of the ROM. By consideration of the overall system and coordination as regards abutment, openings or recesses can be created in the soft biocompatible layer, to increase the ROM.
Optimization of the xe2x80x9cabutmentxe2x80x9d state of operation is possible: This includes adaptation of the abutment geometry of the soft biocompatible layer to minimize the surface pressure that is present and an adaptation of the shaft component to all possible installed positions. These installed positions are defined by different lengths of the neck of the ball heads.
In the event of abutment of the shaft it is possible by appropriate selection of the material of the soft biocompatible layer to assure that very little biocompatible material is released (no metal detritus/no PE flow).
A defined cushioning of the abutment is possible:
By the use of materials with high internal damping qualities and by the functional separation of the bodies in friction and the cushioning element, both internal damping and structural damping elements are possible. Cushioning is achieved in excess of the level of the internal damping by a controlled escape of body fluids (substantially water).
Additional features of the invention will be found in the figures which are described below.